Ed,
Thanks for the many good advice.
I've tried to incorporate as much as possible, updated schematic can be
found here: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
For audio into the PC I'm using a professional balanced microphone to
USB input with a noise level of -130dBc/Hz and no spurs.
Using a 7805 and 1500uF capacitors I tried to create a solid reference
instead of the buffer op amp output and that did make a difference.
Further the input of the first opamp has been change to have identical
resistors at both the + and - input to reduce common mode signals.
None of the capacitors (5.6pF) I tried to reduce high frequency gain
improved the results. Most of the time it got worse.
Removal of a ceramic capacitors eliminated the microphony.
I've added a switch to select between 0dB and 20dB gain so I can
calibrate the level by offsetting the DUT frequency while keeping the
drive to the mixer.
To calibrate the effective noise BW of the FFT I create a test signal
combining a -70dBm 10.001MHz signal with a -90dBm/Hz noise signal.
The 20dB power ratio was confirmed using a calibrated spectrum analyzer.
The FFT length and sample rate at the PC where then changed till the PC
FFT showed the same power ratio.
Noise level at 1kHz is now -150dBc/Hz and -155dBc/Hz at 10kHz which is
the spec of the DOCXO used so no need to go any lower.
Also the 50Hz spurs and its harmonics are greatly reduced.
I'll have to invest in better coax cables as the current cable seem to
leak RF.
Erik.
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